1. Field of the Invention
This invention relates generally to powered surface finishing machines, including walk-behind trowels for finishing concrete surfaces of the type classified in United States Patent Class 404, Subclass 112. More particularly, our invention relates to powered, finishing trowels adapted to accommodate bordering walls or other obstacles, and to guards for such trowels.
2. Description of the Prior Art
It is well established in the concrete finishing arts that freshly placed concrete must be finished properly to achieve the desired flatness. As freshly poured concrete "sets", it soon becomes hard enough to support the weight of powered finishing trowels, that are particularly effective for finishing concrete. Walk-behind trowels are ideal for smaller concrete jobs. They are relatively lightweight and, for most purposes, they can be operated and maneuvered by a single workman. Larger, motorized riding trowels are ideal for finishing much larger areas of plastic concrete.
A typical walk-behind trowel comprises a downwardly projecting rotor that contacts the concrete surface and supports the weight of the trowel. Suitable supporting structure mounts an engine to the rotor assembly for power. In most cases internal combustion motors are preferred, but electric motors can be used with trowels. In many cases the rotor is rotated and coupled to the drive motor by a belt-driven, reduction gearbox. An elongated handle, usually pivoted to the frame, extends angularly rearwardly and terminates in a pair of hand grips that are grasped by the operator to control the trowel. The rotor comprises a plurality of radially spaced-apart finishing blades that revolve in frictional contact with the concrete surface. The rotor blades may be twisted about their longitudinal axes for pitch control. Also, the blades may be coupled to circular finishing pans for treating green concrete. A protective guard cage typically surrounds the rotor assembly to prevent inadvertent blade contact with obstacles or human beings.
Preferably, finishing starts with panning while the concrete is still "green", within one to several hours after pouring depending upon the concrete mixture involved. Pan finishing is followed by blade finishing, after the pan is removed from the rotor. In either case the workman manually directs the trowel over the surface regions to be treated. Trowel movement is aided to some extent by the rotating blades of the rotor. For example, when the operator gently lifts or pushes down on the handle, the trowel will help propel itself in a desired direction because of blade forces on the concrete surface.
However, with known trowels it is difficult to finish concrete regions immediately adjacent walls or other obstacles. Conventional guards that protectively shroud the rotor assembly will contact obstacles or walls if the trowel is moved closely into border regions. The annular region between the outermost periphery of the rotor and the guard ring will not be swept by rotor blades. Further, the many vibrational forces generated by a trowel cause instability and surface marring when a normal fixed guard contacts a wall or obstacle. Even if there is some "give" designed into a trowel guard, normal dynamic forces must be balanced properly for the trowel to treat border regions near obstacles in a stable fashion.
Known walk-behind trowels with fixed guard assemblies are disclosed in prior U.S. Pat. Nos. 3,973,857, 4,320,986, 4,752,154, 4,198,178, 4,232,980, 4,673,311, 4,312,603, 4,046,483, 4,027,991, and 4,629,359. A fixed guard for a walk-behind trowel, owned by Allen Engineering Corporation, is seen in U.S. Des. Pat. No. 400,544. The guards seen in the aforementioned references are fixedly attached to supportive structure and cannot rotate in relation to the trowel.
U.S. Pat. Nos. 5,221,156 and 5,533,830 disclose concrete finishing machines comprising a rotatable, disk-shaped guard that is spaced above the revolving finishing blades. The coaxially aligned guard disk contacts adjacent walls and obstacles, and provides a limited guard function. In these machines the guard disk centers are coaxially coupled to downwardly-projecting drive shaft structure at the center of motor rotation. Suitable bearings facilitate mounting. For clearance purposes, rotor guards secured according to the teachings of the latter patents must be relatively flat and disk-like, and they do not completely, angularly shroud edges of the rotor finishing blades. For example, when a user rocks down on the handle of a walk-behind trowel constructed according to U.S. Pat. Nos. 5,221,156 or 5,533,830, the protective disk will be deflected angularly away from and above the obstacle, leaving the unguarded blade edges to detrimentally make contact. Moreover, relatively small force moments applied to edges of such prior art guards are leveraged into potentially damaging or destabilizing forces applied at the center of rotation, compromising trowel stability and performance. Impact forces experienced by the finishing blades are transmitted directly to the guard through the motor drive shaft. Similarly, impact forces suffered by the guard are transmitted directly to the rotor. An unwanted rocking motion can result, destabilizing the trowel.